The invention relates to bonded abrasive articles or tools, such as grinding wheels, grinding segments, grinding discs and hones, having novel compositional structures, to methods of manufacturing such tools so as to create these novel tool structures, and to methods of grinding, polishing or surface finishing using such tools.
Bonded abrasive tools consist of rigid, and typically monolithic, three-dimensional, abrasive composites in the form of wheels, discs, segments, mounted points, hones and other tool shapes, having a central hole or other means for mounting onto a particular type of grinding, polishing or sharpening apparatus or machine. These composites comprise three structural elements or phases: abrasive grain, bond and porosity.
Bonded abrasive tools have been manufactured in a variety of ‘grades’ and ‘structures’ that have been defined according to practice in the art by the relative hardness and density of the abrasive composite (grade) and by the volume percentage of abrasive grain, bond and porosity within the composite (structure).
For nearly 70 years, tool grade and structure have been considered to be the most reliable predictors of bonded abrasive tool hardness, tool wear rate, grinding power demands, and manufacturing consistency. Grade and structure were first established as reliable manufacturing guidelines in U.S. Pat. No. 1,983,082, to Howe, et al. Howe describes a volumetric manufacturing method useful for overcoming the then persistent difficulties with inconsistent abrasive composite quality and inconsistent grinding performance. In this method, one selects the relative volumetric percentages of the three structural constituents to yield a tool with a targeted grade of hardness and other desired physical characteristics. Knowing the desired volume of the finished tool, the batch weights of abrasive grain and bond components needed to make the tool are calculated from the tool volume, the relative volumetric percentages and the material densities of the abrasive grain and bond components. In this manner it was possible to create a standard structure chart for a defined bond composition and, in subsequent manufacturing runs, to read relative volumetric percentages from the standard structure chart in order to manufacture bonded abrasive tools having a consistent hardness grade for a given volume percentage of abrasive grain, bond and porosity. It was observed that the grinding performance was consistent from one manufacturing batch to another when the grade and structure had been held constant.
For many grinding operations, controlling the amount and type of porosity in the composite, particularly porosity of a permeable, or an interconnected nature, has been shown to improve grinding efficiency and to protect the quality of the work-piece being ground from thermal or mechanical damage.
Any three-dimensional abrasive composite consists of the sum of the relative volume percentages of its three constituents: abrasive grain, bond and porosity. The sum of the volume percentages of these constituents must equal 100 volume percent; therefore, tools having a high percentage of porosity must have proportionally lower percentages of bond and/or abrasive grain. In manufacturing bonded abrasive tools, one can more easily achieve relatively high volume percentages of porosity (e.g., 40-70 volume %) in precision grinding tools, made with rigid, inorganic bond materials (e.g., vitrified or ceramic bonds) and relatively small grain sizes (e.g., Norton grit sizes 46-220 grit), than in rough grinding tools made with organic bond materials and relatively large grain sizes (e.g., Norton grit sizes 12-120 grit). Very porous abrasive composites made with larger grain sizes, higher volume percentages of grain and softer, organic bond materials have a tendency to slump or stratify during the intermediate molding and curing stages of manufacturing the grinding tool. For these reasons, commercially available bonded abrasive tools made with organic bond materials often are molded to contain no porosity, and typically contain no more than 30 volume % porosity. They seldom exceed 50 volume % porosity.
Natural porosity arising from packing of the abrasive grains and bond particles during pressure molding usually is insufficient to achieve high porosity in bonded abrasive tools. Porosity inducers, such as bubble alumina and naphthalene, may be added to abrasive and bond composite mixtures to enable pressure molding and handling of a porous uncured abrasive article and to yield an adequate volume percent porosity in the final tool. Some pore inducers (e.g., bubble alumina and glass spheres) will create closed cell porosity within the tool. Closed cell pore inducers added to achieve high porosity percentages prevent the formation of open channels or interconnected porosity, thus preventing or reducing fluid flow through the body of the tool, thereby tending to increase grinding forces and risk of thermal damage. Open cell pore inducers must be burnt out of the abrasive matrix (e.g., walnut shells and naphthalene), giving rise to various manufacturing difficulties.
Further, the densities of pore inducers, bond materials and abrasive grains vary significantly, making it difficult to control stratification of the abrasive mix during handling and molding, often resulting in a loss of homogeneity in the three-dimensional structure of the finished abrasive article. A uniform, homogeneous distribution of the three constituents of the abrasive composite have been considered a key aspect of consistent tool quality and, for grinding wheels, important in the safe operation of wheels at the high rotational speeds needed for grinding (e.g., over 4000 surface feet per minute (sfpm)).
The volume percent of interconnected porosity, or fluid permeability, has been found to be a more significant determinant of grinding performance of abrasive articles than mere volume percent porosity (see U.S. Pat. No. 5,738,696 to Wu). The interconnected porosity allows removal of grinding waste (swarf) and passage of cooling fluid within the wheel during grinding. The existence of interconnected porosity may be confirmed by measuring the permeability of the wheel to the flow of air under controlled conditions. U.S. Pat. No. 5,738,697 to Wu discloses high permeability grinding wheels having a significant amount of interconnected porosity (40-80%, by volume). These wheels are made from a matrix of fibrous particles having an aspect ratio of at least 5:1. The fibrous particles may be filamentary abrasive grain or ordinary, non-fibrous abrasive grains blended with various fibrous filler materials such as ceramic fiber, polyester fiber and glass fiber and mats and agglomerates constructed with the fiber particles.
It has now been discovered that bonded abrasive tools can be made with a relatively high percentage of porosity and a relatively low percentage of abrasive grain without sacrificing mechanical strength or resistance to tool wear, even though the hardness grade of the tool would predict relatively poor mechanical strength. For organic bonded abrasive tools it is now possible to manufacture tools at relative percentages of abrasive grain, bond and porosity that form structures unknown among commercial bonded abrasives tools. These novel structures include organic bonded abrasive tools wherein the continuous phase of the abrasive composite consists of the porosity constituent. In a preferred method for creating these novel structures, a majority of the abrasive grain has been agglomerated with a binding material prior to mixing, molding and thermally processing the bonded abrasive tool.
Agglomerated abrasive grains have been reported to improve grinding efficiency by mechanisms unrelated to the amount or character of the porosity of the bonded abrasive tool. Abrasive grain has been agglomerated for various purposes, principal among them to allow use of a smaller abrasive grain particle (‘grit’) size to achieve the same grinding efficiency as a larger abrasive grit size, or to yield a smoother surface finish on the workpiece being ground. In many instances abrasive grain has been agglomerated to achieve a less porous structure and a denser grinding tool, having more strongly bonded abrasive grains.
Very low porosity (e.g., less than about 5 volume % porosity) gear honing wheels have been made from reclaimed crushed vitrified bonded abrasive composites by bonding the composites in an epoxy resin. These ‘Compound’ gear honing wheels have been commercially available for a number of years (from Saint-Gobain Abrasives, GmbH, formerly Efesis Schleiftechnik GmbH, Gerolzhofen, Germany).
U.S. Pat. No. 2,216,728 to Benner discloses abrasive grain/bond aggregates made from any type of bond. The reason for using the aggregates is to achieve very dense wheel structures for retaining diamond or CBN grain during grinding operations. If the aggregates are made with a porous structure, then it is for the purpose of allowing the inter-aggregate bond materials to flow into the pores of the aggregates and fully densify the structure during firing. The aggregates allow the use of abrasive grain fines otherwise lost in production.
U.S. Pat. No. 3,982,359 to Elbel teaches the formation of resin bond and abrasive grain aggregates having hardness values greater than those of the resin bond used to bond the aggregates within an abrasive tool. Faster grinding rates and longer tool life are achieved in rubber bonded wheels containing the aggregates.
U.S. Pat. No. 4,799,939 to Bloecher teaches erodable agglomerates of abrasive grain, hollow bodies and organic binder and the use of these agglomerates in coated abrasives and bonded abrasives. Similar agglomerates are disclosed in U.S. Pat. No. 5,039,311 to Bloecher, and U.S. Pat. No. 4,652,275 to Bloecher, et al.
U.S. Pat. No. 5,129,189 to Wetshcer discloses abrasive tools having a resin bond matrix containing conglomerates, having 5-90 vol. % porosity, of abrasive grain, resin and filler material, such as cryolite.
U.S. Pat. No. 5,651,729 to Benguerel teaches a grinding wheel having a core and a discrete abrasive rim made from a resin bond and crushed agglomerates of diamond or CBN abrasive grain with a metal or ceramic bond. The stated benefits of the wheels made with the agglomerates include high chip clearance spaces, high wear resistance, self-sharpening characteristics, high mechanical resistance of the wheel and the ability to directly bond the abrasive rim to the core of the wheel. In one embodiment, used diamond or CBN bonded grinding rims are crushed to a size of 0.2 to 3 mm to form the agglomerates.
GB Pat. No.-A-1,228,219 to Lippert discloses conglomerates of grain and bond added to a rubber, elastic bond matrix. The bond holding the grain within the conglomerate can be ceramic or resin materials, but it must be more rigid than the elastic bond matrix.
U.S. Pat. No. 4,541,842 to Rostoker discloses coated abrasives and abrasive wheels made with aggregates of abrasive grain and a foamed mixture of vitrified bond materials with other raw materials, such as carbon black or carbonates, suitable for foaming during firing of the aggregates. The aggregate “pellets” contain a larger percentage of bond than grain on a volume percentage basis. Pellets used to make abrasive wheels are sintered at 900° C. (to a density of 70 lbs/cu. ft.; 1.134 g/cc) and the vitrified bond used to make the wheel is fired at 880° C. Wheels made with 16 volume % pellets performed in grinding at an efficiency level similar to that of comparative wheels made with 46 volume % abrasive grain. The pellets contain open cells within the vitrified bond matrix, with the relative smaller abrasive grains clustered around the perimeter of the open cells. A rotary kiln is mentioned for firing pre-agglomerated green aggregates that are later foamed and sintered to make the pellets.
U.S. Pat. No. 6,086,467 to Imai, et al, discloses grinding wheels contain abrasive grain and grain clusters of filler grain having a smaller size than the 25 abrasive grain. Vitrified bond may be used and the filler grain may be chromium oxide. The size of the grain clusters is ⅓ or more of the size of the abrasive grain. Benefits include controlled bond erosion and abrasive grain retention in low force grinding applications utilizing superabrasive grain wherein the superabrasive grain must be diluted to minimize grinding forces. Clusters of filler grain may be formed with wax. No sintering of the clusters is disclosed.
WO 01/85393 A1 to Adefris discloses a three-dimensional abrasive article made from abrasive composites, either shaped or irregular, arranged to have more than one monolayer of abrasive composites. The article may contain inter-composite porosity and intra-composite porosity. The composites include abrasive grains bonded in an inorganic or organic first matrix and the abrasive article is bonded with a second inorganic (metal or vitrified or ceramic) or organic binder material, to form an abrasive article having about 20 to 80 volume % porosity. The preferred article contains fine diamond abrasive grain held in a first and a second glass bond and the article is used to grind glass to a mirror finish.
A number of publications have described coated abrasive tools made with agglomerated abrasive grain. They include U.S. Pat. No. 2,194,472 to Jackson which discloses coated abrasive tools made with agglomerates of a plurality of relatively fine abrasive grain and any of the bonds normally used in coated or bonded abrasive tools. Inorganic composites of fine grit diamond, CBN and other thermally degradable abrasive grains in a matrix of metal oxide have been reported to be useful in coated abrasive tools (U.S. Pat. No. 3,916,584 to Howard, et al). U.S. Pat. No. 3,048,482 to Hurst discloses shaped abrasive micro-segments of agglomerated abrasive grains and organic bond materials in the form of pyramids or other tapered shapes. The shaped abrasive micro-segments are adhered to a fibrous backing and used to make coated abrasives and to line the surface of thin grinding wheels. U.S. Pat. No 4,311,489 to Kressner discloses agglomerates of fine (≦200 micron) abrasive grain and cryolite, optionally with a silicate binder, and their use in making coated abrasive tools. U.S. Pat. No. 5,500,273 to Holmes discloses precisely shaped particles or composites of abrasive grits and a polymeric binder formed by free radical polymerization. Similar shaped composites are described in U.S. Pat. No. 5,851,247 to Stoetzel, et al; U.S. Pat. No. 5,714,259 to Holmes, et al; and U.S. Pat. No. 5,342,419 to Hibbard, et al. U.S. Pat. No. 5,975,988, U.S. Pat. No. 6,217,413 B1 and WO 96/10471, all to Christianson, disclose coated abrasive articles include a backing and an organic bonded abrasive layer where the abrasive is present as shaped agglomerates in the shape of a truncated four-sided pyramid or cube.
U.S. Pat. No. 6,056,794 to Stoetzel, et al, discloses coated abrasive articles having a backing, an organic bond containing hard inorganic particles dispersed within it, and abrasive particle agglomerates bonded to the backing. The abrasive particles in the agglomerates and the hard inorganic particles in the organic bond are essential the same size. Agglomerates may be randomly or precisely shaped and they are made with an organic bond. The hard inorganic particles may be any of a number of abrasive grain particles.
U.S. Pat. No. 6,319,108 B1 to Adefris, et al, discloses an abrasive article comprising a rigid backing and ceramic abrasive composites made of abrasive particles in a porous ceramic matrix. The composites are held to the backing with a metal coating, such an electroplated metal. WO 01/83166 A1 to Mujumdar, et al, discloses glass grinding abrasive tools comprising diamond composites held to a backing with resin bond.
A number of patents disclose abrasive tools comprising resin or other organic binder composites of abrasive grain. Most of these tools are coated abrasive tools wherein a resin bond is employed to adhere the abrasive grain composites to a flexible backing. Occasionally metal binders or erodable particles are used in conjunction with the abrasive composites. Representative patents in this group include U.S. Pat. No. 5,078,753 to Broberg, et al; U.S. Pat. No. 5,578,098 to Gagliardi, et al; U.S. Pat. No. 5,127,197 to Brukvoort, et al.; U.S. Pat. No. 5, 318,604 to Gorsuch, et al.; U.S. Pat. No. 5,910,471 to Christianson, et al.; and U.S. Pat. No. 6,217,413 to Christianson, et al.
U.S. Pat. No. 4,355,489 to Heyer discloses an abrasive article (wheel, disc, belt, sheet, block and the like) made of a matrix of undulated filaments bonded together at points of manual contact and abrasive agglomerates, having a void volume of about 70-97%. The agglomerates may be made with vitrified or resin bonds and any abrasive grain. U.S. Pat. No. 4,364,746 to Bitzer discloses abrasive tools comprising different abrasive agglomerates having different strengths. The agglomerates are made from abrasive grain and resin binders, and may contain other materials, such as chopped fibers, for added strength or hardness. U.S. Pat. No. 4,393,021 to Eisenberg, et al, discloses a method for making abrasive agglomerates from abrasive grain and a resin binder utilizing a sieve web and rolling a paste of the grain and binder through the web to make worm-like extrusions. The extrusions are hardened by heating and then crushed to form agglomerates.
Notwithstanding this extensive body of knowledge regarding how to make abrasive articles with agglomerated grain and to eliminate or create tool porosity, until now, no one has successfully altered the basic composite structure of a three-dimensional, monolithic bonded abrasive tool with agglomerated grain such that tool grade and structure no longer predict grinding performance. No one has utilized agglomerated grain to make volume percent structure tools that were difficult or impossible to manufacture with ordinary abrasive grain in organic bonds. In particular, without sacrificing mechanical strength, tool life or tool performance, it has been found that relatively high volume percentages of porosity (e.g., above 30 volume %) may be achieved in bonded abrasive tools made with organic bonds. Significant alterations in elastic modulus and other physical properties of both inorganic and organic bonded tools now can be achieved in the tools of the invention.
In bonded abrasives made with organic bond materials, the bond materials have been considered to be the most important factor in altering the grade and structure to achieve appropriate or sufficient mechanical strength or rigidity. Quite surprisingly, the invention permits lower abrasive grain content tools to be made over a range of bond contents and used in grinding applications that demand high mechanical strength tools having resistance to premature wear (defined as tool structure wear that is more rapid than abrasive grain wear). In large contact area surface grinding applications, the tools of the invention actually perform in a manner superior to conventional tools made with higher bond and abrasive grain contents.
None of the prior art developments in agglomerated abrasive grain suggest the benefits in bonded abrasive tools of using certain, agglomerated abrasive grains within an organic or inorganic bond matrix to control the three-dimensional structure of the bonded abrasive tool. In particular, it is unexpected that these agglomerates could be adapted to tailor and to control the location and type of porosity and bond matrix within the structure of the tools of the invention.